Could simply be air in the system. Nearly every hydraulic leveler system has a procedure for bleeding air.
Before panicking about leaks, etc. try COMPLETELY extending all four jacks. Extend the fronts first, then the rear. Let them sit FULLY extended for an hour or so. Then retract all four jacks.
Most jacks have a small plunger type switch at the top of each jack to signal they are fully retracted. Air in the system may prevent one or more jacks from fully retracting, preventing the switch from being triggered - so the system "thinks" one of the jacks is extended.
If able, crawl under and look at the top part of each jack. You may be able to see a small plunger switch. See if it's FULLY depressed by the jack foot. If one of the feet is down a little, and not fully pressing that switch - that might be your problem. Air in the system may be the issue.
NO GUARANTEES... but worth checking.